XNA IScreen code

using Microsoft.Xna.Framework;

namespace IScreenSystem
{
    public interface IScreen
    {
        void Update(GameTime gameTime);
        void Draw(GameTime gameTime);
    }
}

using Microsoft.Xna.Framework;

namespace IScreenSystem
{
    static class ServiceExtensionMethods
    {
        public static void AddService<t>
            (this GameServiceContainer services, T service)
        {
            services.AddService(typeof(T), service);
        }

        public static T GetService<t>(this GameServiceContainer services)
        {
            return (T)services.GetService(typeof(T));
        }
    }
}

using Microsoft.Xna.Framework;
using Microsoft.Xna.Framework.Graphics;
using Microsoft.Xna.Framework.Content;

namespace IScreenSystem
{
    public class VariableService
    {
        public Game Game { get; set; }
        public GraphicsDeviceManager Graphics { get; set; }
        public GraphicsDevice GraphicsDevice { get; set; }
        public ContentManager Content { get; set; }
        public SpriteBatch SpriteBatch { get; set; }
        public IScreen CurrentScreen { get; set; }
    }

}

using System;
using System.Collections.Generic;
using System.Linq;
using Microsoft.Xna.Framework;
using Microsoft.Xna.Framework.Audio;
using Microsoft.Xna.Framework.Content;
using Microsoft.Xna.Framework.GamerServices;
using Microsoft.Xna.Framework.Graphics;
using Microsoft.Xna.Framework.Input;
using Microsoft.Xna.Framework.Media;

namespace IScreenSystem
{
    class TestScreen : IScreen
    {
        private VariableService vars;

        public TestScreen(Game game)
        {
            vars = ServiceExtensionMethods
                .GetService<variableservice>(game.Services);
        }

        public void Update(GameTime gameTime)
        {
            // to change screen, simply
            // vars.CurrentScreen = new TestScreen(vars.Game);
        }

        public void Draw(GameTime gameTime)
        {
            vars.GraphicsDevice.Clear(Color.Black);
        }

    }
}

using Microsoft.Xna.Framework;
using Microsoft.Xna.Framework.Graphics;

namespace IScreenSystem
{
    public class Game1 : Microsoft.Xna.Framework.Game
    {
        private VariableService vars;
        private GraphicsDeviceManager graphics;

        public Game1()
        {
            graphics = new GraphicsDeviceManager(this);
            Content.RootDirectory = "Content";
        }

        protected override void Initialize()
        {
            base.Initialize();
        }

        protected override void LoadContent()
        {
            ServiceExtensionMethods.AddService<variableservice>
                (this.Services, new VariableService());

            vars = ServiceExtensionMethods
                .GetService<variableservice>(this.Services);
            vars.Game = this;
            vars.Content = Content;
            vars.Graphics = graphics;
            vars.GraphicsDevice = graphics.GraphicsDevice;
            vars.SpriteBatch = new SpriteBatch(GraphicsDevice);
            vars.CurrentScreen = new TestScreen(this);
        }

        protected override void UnloadContent() { }

        protected override void Update(GameTime gameTime)
        {
            vars.CurrentScreen.Update(gameTime);
            base.Update(gameTime);
        }

        protected override void Draw(GameTime gameTime)
        {
            GraphicsDevice.Clear(Color.CornflowerBlue);
            vars.CurrentScreen.Draw(gameTime);
            base.Draw(gameTime);
        }
    }
}